(a) Field of the Invention
The present invention relates to an adjustable diopter type finder optical system.
(b) Description of the Prior Art
Generally, with finders of optical devices, binoculars, microscopes, etc., an image formed at a predetermined position by the optical system is observe through an eyepiece. When, at that time, the image is formed at a position out of the range wherein the image can be seen by the adjusting function of eye, it is difficult to observe the image. To observe the image formed at such position, it is necessary to use a diopter adjusting means which enables to adjust the image to a position where it can be seen easily, according to the adjusting ability of eye. For example, in a known finder optical system for a single-lens reflex camera provided with the above-mentioned diopter adjusting means, it is so arranged that the eyepiece constituting the optical system can be moved forward and backward in respect to the image formed by the finder (finder image). However, in the known finders of single-lens reflex cameras of 35 mm format, a pentagonal prism exists between the finder and eyepiece and, therefore, the focal length f.sub.e of the eyepiece necessarily becomes large, i.e., about 60 mm. Consequently, to obtain the desired variation of diopter, the movement amount of the eyepiece becomes large. The movement amount .DELTA. of the eyepiece per 1D (diopter) is given by the following formula. EQU .DELTA.=(f.sub.e.sup.2 /1000)
Therefore, to vary the diopter by 2D, it is necessary to move the eyepiece by about 7 mm. However, in case of single-lens reflex cameras of 35 mm format, the eyepiece should be arranged in a very limited space behind the pentagonal prism and, therefore, it is not possible to ensure a sufficient space which enables to move the eyepiece by a large amount as described in the above.
To obtain the desired variation of diopter by a small movement amount of eyepiece, it is known to arrange a finder optical system so that its eyepiece comprises two lens components, i.e., a negative lens and positive lens, and the diopter is adjusted by keeping either one of the negative lens and positive lens fixed and moving the other one only. With the above-mentioned optical system, it is possible to obtain a large variation of diopter by a very small movement amount by arranging that the focal length of the movable lens in the eyepiece is made shorter than the focal length of the eyepiece as a whole. However, to obtain the predetermined value of focal length of the eyepiece as a whole with such eyepiece that the focal length of the movable lens is made shorter than the focal length of the eyepiece as a whole, the focal length of the fixed lens should be also made short. When a lens with a short focal length is moved as described in the above, disadvantages are caused as explained below.
A first disadvantage is that the magnification of the finder optical system varies according to the movement of the movable lens. For example, for single-lens reflex cameras, the magnification of the finder optical system is generally expressed by the magnification .beta. of the telescope formed by the photographing lens system and eyepiece as shown by the following formula, where reference symbol f.sub.T represents the focal length of the photographing lens system. EQU .beta.=(f.sub.e /f.sub.T)
Therefore, when the diopter is adjusted by moving the eyepiece as a whole, the magnification of the finder optical system does not change, even when diopter adjustment is made, because f.sub.e and f.sub.T do not change. When, however, the eyepiece comprises two lens components, i.e., a negative lens and positive lens, and the diopter is adjusted by moving one of those lenses, the airspace between the two lenses varies at the time of diopter adjustment and, consequently, the focal length f.sub.e of the eyepiece varies. Therefore, when the focal length f.sub.T of the photographing lens system does not vary, the magnification of the finder optical system varies according to adjustment of diopter. Variation of f.sub.e becomes larger when the focal lengths of the two lenses are shorter and when the variation in the relative positions of the two lenses is larger.
A second disadvantage is that aberrations of the eyepiece fluctuate according to the movement of the movable lens. For a lens system with simple lens configuration such as an eyepiece, aberrations of the eyepiece as a whole are generally made small by arranging so that aberrations caused by the negative lens are cancelled by the positive lens. The above-mentioned offsetting of aberrations of one lens by the other lens is possible only when the two lenses are kept in the predetermined relative positions. If the relative positions become different, aberrations of one lens cannot be satisfactorily offset by the other lens and, consequently, aberrations of the eyepiece as a whole become unfavourable. In other words, when one lens is moved for adjustment of diopter, aberrations of the eyepiece fluctuate. This fluctuation of aberrations also become larger when the focal lengths of respective lenses are shorter and when the variation in the relative positions of those lenses is larger.